Production of p-nu junctions by diffusion



United States Patent 3,351,503 PRODUCTION OF P-N JUNCTIONS BY DIFFUSIONRichard A. Fotland, Lyndhurst, Ohio, assignor to Horizons incorporated,a corporation of New Jersey No Drawing. Filed Sept. 10, 1965, Ser. No.486,550

4 Claims. (Cl. 148-488) ABSTRACT OF THE DESCLCSURE This inventiondescribes the production of P-N junctions on a semiconductor body bydecomposing an adsorbed film of dopant, existing on the surface of thebody, through the action of an electron beam.

This invention relates to the formation of P-N junctions. Moreparticularly, it relates to the production of P-N junctions on asemiconductor body without recourse to any of the techniques heretoforeused in such manufacture.

One presently known technique for fabricating discrete area P-Njunctions on silicon or germanium involves a sequence of steps includingcovering the surface of a semiconductor chip with a photoresist,exposing the photoresist by contact printing through a lithographic typenegative, processing the photoresist, treating the semiconductor surfaceto form a barrier layer (in the case of silicon this layer is generallysilicon dioxide), removing the photoresist from the surface, and formingthe junction by diffusion or epitaxial growth techniques well known tothose versed in the art. This process is both time consuming and resultsin low yields because of the large number of processing steps involved.

The present invention overcomes the requirements for masks by definingthe P-N junction area on a semiconductor slab simply by scanning anelectron beam over the surface of the slab under suitable conditions. Noart work or resists are required and the area on the surface of thesemiconductor slab, which is converted to form a P-N junction, isdefined by electrical deflection signals used to control the position ofthe electron beam. The electron beam scan may be readily controlled bycomputer, punch cards, magnetic tape and the like in order to generatecomplex patterns of P-N junctions on the semiconductor surface.

In accordance with the present invention, P-N junctions of definiteconfigurations are produced on semiconductor base materials by utilizingan electron beam to decompose an adsorbed vapor of a compound of adesired dopant, and at the same time effect diffusion of the de siredatoms into the semiconductor material.

A preferred procedure for forming a P-N junction at the surface of asilicon or germanium slab comprises the following sequence of steps:

Slicing the slab to the proper size;

Etching the surface to prepare a clean surface on the slab;

Mounting the clean slab in an evacuated chamber positioned so as to beat the focal plane of a low energy electron beam, then evacuating thevacuum chamber;

Introducing a dopant into the system at a pressure of between and 10*torr;

Generating an electron beam using a conventional electron gun consistingof a cathode, accelerating electrodes, focusing electrodes or magneticfocusing coil and appropriate deflection coils;

Focusing the beam at the surface of the semiconductor slab; and

Scanning the area which is to be converted, thereby forming a P-N or N-Pjunction.

Preferred dopants include arsenic trichloride for the formation ofP-type surface layers and boron trichloride for the preparation ofN-type surface regions. Upon the introduction of either of these vaporsinto the system, a monolayer of dopant molecule is adsorbed at thesurface of the semiconductor. If P-type silicon is employed as a slab,arsenic trichloride is introduced in order to convert the surface, uponelectron beam irradiation, to

-type. Similarly, if N-type silicon is employed, boron trichloride vaporis introduced. The electron beam interacts with adsorbed molecules,decomposing the adsorbed molecule possibly by direct interaction of theelectron beam with a dopant atom or as a result of the intense localheat generated instantaneously at the surface by the bombarding beam.This intense heat also provides the thermal activation energy fordiffusing the dopant atom into the surface of a semiconductor. of thisinvention, a slab of P-type silicon a vacuum system and a small area onthe surface scanned with a 500 volt electron beam at a beam current ofmicroamperes for a period of two hours. During this time, arsenictrichloride vapor was maintained in the chamber at a pressure of l 10torr. The area irradiated by the electron beam was converted to a P-Njunction. The converted surface was electroded with evaporated aluminum.The diode thus formed by the was evaluated and it was found that theback-to-forward resistance ratio at 1 volt was slightly over 1000. Thepeak inverse voltage of this diode was 8 volts.

It is also possible to carry out the invention using a P-type dopant andan N-type base, for example with N-type silicon as the base and borontrichloride as the boron contributing dopant. Using the same operatingconditions as in the preceding example diodes were formed havingback-to-forward resistance ratios of about 800 and a peak inversevoltage breakdown of 7 volts.

This technique has the advantage of allowing P-N junctions to be formedat preselected discrete positions upon silicon or germanium slabswithout resorting to the multiple steps involved in the conventionalphotoresist technique.

Although gas plating of metals at elevated temperatures and the electronbeam decomposition of adsorbed compounds are well-known to those skilledin the art, the formation of P-N junctions, involving, as it does, thediffusion of impurities into a semiconductor lattice employing anelectron beam is not obvious from the literature. In the electron beamformation of P-N junctions described herein, the electron beam serves toboth decompose the adsorbed metal halide and to cause the metal atom todiffuse into the semiconductor structure.

A related invention is described in my copending application Serial No.184,995 filed April 4, 1962, now abandoned.

Having now described preferred embodiments of the invention it is notintended that it be limited except as may be required by the appendedclaims.

Iclaim:

1. A method of producing P-N junctions which comprises: positioning asemiconductor material selected from the group consisting of P-typesilicon and N-type silicon in an atmosphere of a decomposable vapor of adopant compound at a pressure such that a layer of the vapor is adsorbedon the surface of the semiconductor material; decomposing selectedportions of said adsorbed layer by scanning the same with an electronbeam, thereby was mounted in depositing a dopant on said semiconductorsurface and diffusing the same into said semiconductor material, therebyproducing a P-N junction in said semiconductor material.

2. The method of claim 1 wherein the dopant is arsenic trichloride whenthe semiconductor body is P-type silicon.

3. The method of claim 1 wherein the dopant is boron trichloride whenthe semiconductor body is N-type silicon.

4. The method of claim 1 wherein the pressure is between about 10' and10- torr.

References Cited UNITED STATES PATENTS Steigerwald.

Derick 148-189 Ligenza 148187 Quinn 148-1.5 Hora 1481.5X

l0 HYLAND BIZOT, Primary Examiner.

1. A METHOD OF PRODUCING P-N JUNCTIONS WHICH COMPRISIES: POSITIONING ASEMICONDUCTOR MATERIAL SELECTED FROM THE GROUP CONSISTING OF P-TYPESILICON AND N-TYPE SILICON IN AN ATMOSPHERE OF A DECOMPOSABLE VAPOR OF ADOPANT COMPOUND AT A PRESSURE SUCH THAT A LAYER OF THE VAPOR IS ADSORBEDON THE SURFACE OF THE SEMICONDUCTOR MATERIAL; DECOMPOSING SELECTEDPORTIONS OF SAID ADSORBED LAYER BY SCANING THE SAME WITH AN ELECTRONBEAM, THEREBY DEPOSITING A DOPANT ON SAID SEMICONDUCTOR SURFACE ANDDIFFUSING THE SAME INTO SAID SEMICONDUCTOR MATERIAL, THEREBY PRODUCING AP-N JUNCTION IN SAID SEMICONDUCTOR MATERIAL.